1. Field of the Invention.
The present invention relates to a method of synthesizing isocyanate prepolymers for use in polyurethane synthesis. More particularly, the invention relates to the synthesis of isocyanate prepolymers from epoxidized natural oils and polyisocyanates.
2. Prior Art.
Polyurethanes comprise a wide variety of polymers. They are, in fact, the most ubiquituous polymer in the chemical industry today. Polyurethanes are defined as any polymer formed by linking isocyanic groups to hydroxyl groups present on different monomers. The polymer-forming hydroxyl and isocyanic groups can be attached to an endless variety of monomers, making the varieties of polyurethanes equally endless. Polyisocyanates and polyhydroxyl compounds may also be used to increase crosslinking in polyurethanes. Not surprisingly, uses for polyurethanes are almost as endless as their varieties. Polyurethane is used for adhesives, sealants, coatings and elastomers. The multiple uses and varieties of polyurethanes are well known in the art.
Isocyanates readily react with water, and are severe eye and skin irritants. When inhaled, they can be toxic to humans. A significant amount of isocyanate vapor is released during the synthesis of polyurethane. This poses a significant danger to humans. In order to decrease the risk of harm to persons working with polyurethanes, it has become well known in the art to use a safer isocyanate compound, known as a prepolymer, in the polyurethane polymerization reaction. Prepolymers are formed by reacting isocyanates or polyisocyanates with another compound such as a diol or an epoxide. The resulting prepolymer binds to the isocyanate and prevents the release of harmful vapors during the polymerization reaction. This safer prepolymer is then used to form polyurethane.
Epoxides are readily available, and their use in formation of prepolymers is well known in the art. When isocyanates are reacted with epoxides, prepolymers containing oxazolidone rings are formed. These oxazolidone rings not only prevent evaporation of the isocyanates, but also provide favorable thermal qualities to the resulting polyurethane. Oxazolidones may also be polymerized themselves, forming oxazolidone polymers or even oxizolidone/polyurethane combination polymers. These combinations provide for even greater variety and versatility of polyurethane chemistry. They are also well known in the art.
Reactions between monoepoxides and monoisocyanates are fairly straight forward. The reactants are heated in the presence of a catalyst, and the reaction runs to completion. However, the reactions between polyepoxides and polyisocyanates are much more complex. These reactions produce a variety of by-products that are deleterious to the final polymer. Polyisocyanates form trimers known as isocyanurates. These trimers increase the viscosity of the prepolymer and contribute to brittleness of the final product. Homopolymerization of polyepoxides results in polyethers that also reduce the quality of the final product. In addition, both of these reactions compete with the favorable oxazolidone forming reaction. The quantity of oxazolidones produced can be increased by modifying the reaction temperature and using specific catalysts. It is known in the art that various catalysts, such as halide compounds, organometal compounds, quaternary ammonia salts and organoantimony iodine, may be used to encourage the desired reaction and inhibit the formation of isocyanurate and polyethers.
The majority of epoxides used to form isocyanate prepolymers are synthetic, often derived from petrochemicals. These compounds are generally harmful to both the environment and those handling them. Safety has always been a major concern in the chemical industry. In recent years environmental concerns have also been addressed by the industry. There is an active trend toward using renewable, natural resources. This benefits the environment, and can sometimes be less expensive than using synthetic components. Natural oils, derived from plants and animals, exist in great abundance. They are readily available, inexpensive and very renewable. In addition they are easily epoxidized to form polyepoxides. As these oils are abundant, natural and renewable, it is desirable to use their epoxides in the formation of isocyanate prepolymers. Unfortunately, natural oils tend to contain ester groups. These ester groups may, under certain conditions, be hydrolyzed, decomposed or transesterified. This leads to a plethora of functional groups. When reacted with isocyanates, these additional functional groups interfere with the formation of the desired product, oxazolidones. They also cause side reactions that lead to the formation of a variety of undesirable by-products. Therefore, epoxidized natural oils have been considered poor epoxides for the formation of isocyanate prepolymers.
It is therefore desirable to develop a process whereby natural oil epoxides may by used to form isocyanate prepolymers.
It is also desirable to develop a fast, simple process for forming stable isocyanate prepolymers having a long shelf life.
It is also desirable to provide a new method for making sealants by using epoxidized natural oil based isocyanate prepolymers as the raw material.
It is also desirable to provide a new method for making coatings by using epoxidized natural oil based isocyanate prepolymers as the raw material.
It is also desirable to provide a new method for making adhesives by using epoxidized natural oil based isocyanate prepolymers as the raw material.
It is also desirable to provide a new method for making elastomers by using epoxidized natural oil based isocyanate prepolymers as the raw material.
The method of the present invention involves converting epoxidized natural plant or animal oils to isocyanate prepolymers by reaction with polyisocyanates. The reaction product comprises a mixture of chemicals including oxazolidones, urethanes, amides, urea, isocyanurates, uretdiones and esters. Oxazolidone is the dominant product of the reaction. This method takes place at atmospheric pressure.
The process of the present invention comprises mixing an epoxidized natural oil in solution with isocyanate and a catalytic amount of a Lewis acid so as to form a natural oil-based isocyanate prepolymer. An organic solvent, isocyanate and a Lewis acid are poured into a reaction vessel. The content is stirred and heated to 150-160xc2x0 C. The epoxidized natural oil is then added. The reaction temperature is increased up to the boiling point of the solvent and maintained at that temperature until the completion of the reaction. The progress of the reaction may be monitored by titration. The reaction is finished when the isocyanate concentration drops to about half of the starting concentration. The reaction generally takes anywhere from 10 to 180 minutes.
Any epoxidized natural or animal oil may be used in this process. Oils extracted from vegetables, flowers, seeds and animals are all suitable for use in the present invention. These oils are both renewable and inexpensive. They are also readily commercially available in epoxidized form. The iodine values of these natural oils range from about 40 to 220 and more preferably from about 80 to 180.
Any polyisocyanate may be used. Some polyisocyanates react more quickly and/or at lower temperatures than others. This is generally due to the geometric configuration of the molecule itself. The presence of conjugated rings, aliphatic groups and steric hindrances will all affect the reactivity of an individual isocyanate. The isocyanate chosen will depend not only on its reactivity, but the use to which the prepolymer and resulting polyurethane will be put.
The catalyst may be an organometal compound, a Lewis acid or a halide compound. Preferably, aluminum trichloride is the catalyst used. Aluminum trichloride provides for a shorter reaction time. The natural oil-based prepolymer is also produced more consistently, and has a lower viscosity and longer stability when aluminum chloride is used.
The solvent should have a relatively high boiling point, preferably between 140 to 200xc2x0 C. The solvent can not have any hydrogen donor functional groups. Appropriate solvents will also not react with isocyanate or epoxidized oil at elevated temperatures.
Natural oils contain fatty ester chains. Epoxy groups located on these chains tend to be less reactive. Therefore, higher reaction temperatures are usually required when using natural oils having fatty ester chains. Additional time may also be required to complete reactions with these natural oils.
The prepolymers formed are characterized by the presence of one or more oxazolidone rings. The oxazolidone rings formed will react with compounds having hydroxyl groups to form polyurethane. It is often desirable to increase the amount of crosslinking by using a polyol.
The prepolymers formed by the present invention may further be reacted with polyols to form polyurethanes. By using different polyols, polyurethanes having different qualities are produced. The present invention further provides methods for producing distinct polyurethanes, each developed for a particular purpose. Specifically, polyurethanes are provided that are suitable coatings, sealants, adhesives and elastomers. These polyurethanes provide suitable alternatives to ones derived from synthetic epoxides. Such compositions are highly desirable because they utilize renewable, inexpensive base materials. It may also prove beneficial in some applications to form oxazolidone polymers or a oxazolidone/polyurethane combination polymers.
It is therefore an object of the present invention to provide a process for synthesizing isocyanate prepolymers from epoxidized natural oils.
It is a further object of the present invention to provide a fast simple process for forming stable isocyanate prepolymers having a long shelf life.
It is a further object of the present invention to provide a new method for making sealants by using epoxidized natural oil based isocyanate prepolymers as the raw material.
It is another object of the present invention to provide a new method for making coatings by using epoxidized natural oil based isocyanate prepolymers as the raw material.
It is a further object of the present invention to provide a new method for making adhesives by using epoxidized natural oil based isocyanate prepolymers as the raw material.
It is a further object of the present invention to provide a new method for making elastomers by using epoxidized natural oil based isocyanate prepolymers as the raw material.
The method of the present invention involves converting epoxidized natural plant or animal oils to isocyanate prepolymer by reaction with polyisocyanates. The reaction product comprises a mixture of chemicals including oxazolidones, urethanes, amides, urea, isocyanurate, uretdiones and esters. Oxazolidones are the dominant product of the reaction. This method takes place at atmospheric pressure.
The process of the present invention comprises mixing an epoxidized natural oil in solution with isocyanate and a catalytic amount of a Lewis acid so as to form a natural oil-based isocyanate prepolymer. An organic solvent, an isocyanate and a Lewis acid are poured into a reaction vessel. The contents are stirred and heated to 150-160xc2x0 C. The epoxidized natural oil is then added. The reaction temperature is increased up to the boiling point of the solvent and maintained there until the reaction is complete. The progress of the reaction may be monitored by titration. A sample of the reaction is removed and titrated for isocyanate content. On a larger scale, other methods of measuring isocyanate concentration may prove more practical or efficient. The reaction is finished when the isocyanate concentration drops to about half of the starting concentration. Depending on the use the prepolymer is put to, it may be desirable to extract and purify the oxazolidone product from the reaction mixture.
Any epoxidized natural plant or animal oil may be used in this process. Examples of natural oils that may be used include, but are not limited to, soybean oil, safflower oil, linseed oil, corn oil, sunflower oil, olive oil, canola oil, sesame oil, cottonseed oil, palm oil, rapeseed oil, tung oil, and fish oil. Any partially hydrogenated and epoxidized natural or genetically modified natural oil can be used in this process. These oils include, but are not limited to, high oleic safflower oil, natural oil, peanut oil, sunflower oil (NuSun sunflower oil) and high erucic rapeseed oil (Crumbe oil). Fish oil among animal oils can be also used. The iodine values of these natural oils range from about 40 to 220 and more preferably from about 80 to 180.
Examples of isocyanates that may be used in the prepolymerization reaction include, but are not limited to aromatic isocyanates such as 4,4xe2x80x2-diphenylmethane diisocyanate (MDI), any polymeric MDI such as PAPI (a product of the Dow Chemicals Company), Isonate (also a product of the Dow Chemicals Company), Mondur (a product of the Bayer Company) or other brand names polymeric MDI isocyanates, any MDI based prepolymer, 2,4:2,6-toluene diisocyanate as an 80:20 or 65:35 mixture, pure 2,4-toluene diisocyanate, pure 2,6-toluene diisocyanate, any TDI-based prepolymer, p-phenylene diisocyanate, 1,5-naphthalene diisocyanate, 4,6-xylylene diisocyanate, 3,3xe2x80x2-tolidine-4,4xe2x80x2-diisocyanate, 3,3xe2x80x2-dimethyl-diphenylmethane 4,4xe2x80x2-diisocyanate, aliphatic isocyantes such as 1,6-hexamethylene diisocyanate, or 2,2,4-(2,4,4-) trimethylhexamethylene diisocyanate, cycloalipahtic isocynates such as 4,4xe2x80x2-dicyclohexylmethane diisocyanate, isophorone diisocynate or cyclohexyl diisocynate, or any isocyanate prepolymer. The isocyanate used in any particular prepolymer will be dictated by its reactivity, the use it is to be put to (coating, sealant, adhesive, elastomer, etc.) and other factors known to those skilled in the art.
The catalysts used in the present invention may be organometal compounds such as triethyl aluminum, triisopropyl aluminum, triphenylantymone iodide, or Lewis acids such as aluminum trichloride, zinc iodide and lithium chloride. The most preferable is aluminum trichloride. Use of aluminum trichloride as the catalyst shortens the reaction time and produces a natural oil-based prepolymer that has a lower viscosity and longer stability. Use of aluminum trichloride also discourages the formation of isocyanurate and other unwanted by-products. The catalytic amount of aluminum trichloride is used in this reaction, which should be below 0.5% by weight of the complete reaction mixture, or 1.5% by weight of the amount of epoxidized oil used.
The solvents used in the present invention are organic solvents having boiling points in the temperature range of 140 to 200xc2x0 C. The solvents must not have any hydrogen donating functional groups and must not be reactive toward isocyanate or epoxidized oil. Exemplary solvents include xylene and 2-ethoxyethyl acetate. However, any other non-reactive solvent capable of substantially dissolving the reactive compounds (isocyanates and epoxidized oils) is sufficient. In the following exemplary embodiments, solvents were used at a concentration between 0-30% of the total reaction mixture, preferably 10-20%. However, other suitable solvents may provide an adequate reaction solution when present in a substantially greater or lesser concentration.
The reaction time is in a range between 10 to 180 minutes and depends on used isocyanate. Aromatic isocyanates are more reactive and reaction time is between 30-60 minutes. Aliphatic, cycloaliphatric and sterically hindered isocyanates require longer reaction time, up to 180 minutes. The reaction is sustained until the isocyanate concentration has dropped by about 50%.
The location of the oxirane groups on the epoxidized natural oils may affect the speed and efficiency of the reaction. Epoxy groups located in the middle of fatty ester chains found in some natural oils have relatively low reactivity. These oils may require more vigorous reaction conditions, such as higher temperature, to push the reaction equilibrium toward the oxazolidone product.
Once the prepolymers are formed, they may be used in the formation of various polyurethanes. This invention provides for the use of these polyurethanes as coatings, sealants, adhesives and elastomers. However, this invention may also be used to form polyurethanes for other purposes. The invention further anticipates the use of the described prepolymers to form oxazolidone polymers or oxazolidone/polyurethane polymer combinations. Once the prepolymer synthesis reaction is complete, it may be desirable to extract and purify the oxazolidones from the reaction mixture. For some uses, however, it may be desirable to use the reaction mixture itself in polyurethane synthesis. Polyurethane foams may be formed by retaining the unreacted isocyanates. This isocyanate may react with water during polyurethane synthesis, forming a gas and creating bubbles. This results in polyurethane foam. Depending on the isocyanates and polyols used, these foams may be sturdy or flexible, and of varying strength and density.